1. Field of The Invention
The invention relates to aluminum-transition metal-silicon alloys produced by the carbothermic reduction of aluminous ores containing silica, and metal oxides, such as iron or titanium oxides. More particularly, the invention relates to carbothermically reduced aluminum-iron-silicon alloys that have been rapidly solidified from the melt and thermomechanically processed into structural components having a combination of high ductility (toughness) and high tensile strength.
2. Brief Description of the Prior Art
P. Van Mourik, et al. in the article "On Precipitation in Rapidly Solidified Aluminum-Silicon Alloys", Journal of Materials Science 18 (1983), pp. 2706-2720; discusses the precipitation of Si in rapidly solidified Al-Si alloys. The alloys were prepared by mixing selected proportions of substantially pure Al and Si, and then melt spinning the molten alloys compositions at a quench rate ranging from 10.sup.6 to 10.sup.7 K/sec, as particularly discussed at page 2707 thereof.
R. O. Suzuki, Y. Komatsu, K. F. Kobayashi, P. H. Shingu ("Al-Fe-Si Alloys", Journal Materials Science, vol. 18, 1983 pp. 1195-1201) have investigated amorphous Al-Fe-Si alloys produced by the gun method and by single roller quenching. Specifically, compositions near the .beta.-Al.sub.9 Fe.sub.2 Si.sub.2 intermetallic compound (Al--13 wt. % Fe--17.4 wt. % Si) were the only aluminum-iron-silicon compositions which could be quenched into the amorphous state at cooling rates of 10.sup.5 -10.sup.6 K/sec. No consolidation data or mechanical properties were reported for the alloys discussed in this paper.
The Bayer and Hall-Heroult processes for the extraction of alumina from bauxite, and the production of liquid aluminum by electrolysis of alumina has been the main-commercial process for producing aluminum. Extensive work has been carried out by the major aluminum companies on alternative production methods, using carbothermic reduction of aluminosilicate ores, and electrolysis of refined aluminum chloride. Both processes have been widely researched and numerous patents have been issued, for example:
1. U.S. Pat. No. 3,661,561 "Method of Making Aluminum Silicon Alloys" to F. W. Frey, et al. PA0 2. U.S. Pat. No. 3,661,562 "Reactor and Method of Making Aluminum-Silicon Alloys" to K. K. Seth, et al. PA0 3. U.S. Pat. No. 3,758,289 "Prereduction Process" to J. W. Wood. PA0 4 . U.S. Pat. No. 3,758,290 "Carbothermic Production of Aluminum" to R. M. Kirby. PA0 5. U.S. Pat. No. 4,046,558 "Method for the Production of Aluminum-Silicon Alloys" to S. K. Das, and R. A. Milito et al. PA0 6. U.S. Pat. No. 4,053,303 "Method of Carbothermically Producing Aluminum-Silicon Alloys" to C. N. Cochran, et al.
Efforts directed to the commercial carbothermic melting of aluminum have been reviewed by P. T. Stroup in 1964 (Trans. Met. Soc.) AIME, Vol. 230, pp. 356-372.
There have been systematic investigations of the production of pure aluminum from various ores ranging from bauxite (50% Al.sub.2 O.sub.3, 15% Fe.sub.2 O.sub.3, 2% SiO.sub.2), which has highest available alumina content, to various clays and feldsparthic decomposition weathering products, which have generally higher silica and iron oxide contents and lower alumina contents. In general, reduction to pure aluminum is the most difficult carbothermic reaction, with reduction to aluminum-silicon alloys having more attractive reaction kinetics. During the 1960's, for example, Reynolds Aluminum operated a 2 MW pilot plant producing aluminum-silicon alloys from the carbothermic reduction of nephaline ores containing 25% Al.sub.2 O.sub.3, 50% SiO.sub.2, 2% Fe.sub.2 O.sub.3. It has generally been considered that the carbothermic reduction reactions proceed at somewhat lower temperatures when silicon is present, although the understanding of the direct reactions involved are considered somewhat speculative by Stroup.
The presence of iron oxides in the initial ore results in iron being present in the final alloy. As discussed by Das and Milito (U.S. Pat. No. 4,046,558), the presence of iron produces higher product yields by lowering the volatility of aluminum rich reaction products. Das, et al. discuss a method of carbothermic reduction of natural lateritic ores, and synthetic ore mixtures having widely differing chemistries (15-48 wt. % Al.sub.2 O.sub.3, 2-68 wt. % SiO.sub.2, and 3.8-60 wt. % Fe.sub.2 O.sub.3). The resultant aluminum-silicon alloys contain unspecified quantities of iron.
Fujishige, et al. (Journal Japanese Inst. Met., Dec. 1983, 47(12), p. 1047-1054) have described carbothermic reduction of aluminous ores with high temperature carbon monoxide, and concluded that bauxite ores with high iron contents represented the most favorable raw materials for carbothermic reduction in a blast furnace.
Kuwahara in U.S. Pat. No. 4,394,167 discloses a method for producing aluminum metal in which alumina, silica and oxide bearing materials are mixed with coal. The mixture is heated to produce alumina bearing, coked briquettes. Then, the coked briquettes are brought to a temperature ranging from 2,000.degree. to 2,100.degree. C. to produce an aluminum, silicon and iron containing alloy. The alloy is scrubbed by a molten lead spray directly after the alloy formation, and converted to a lead-aluminum alloy. Aluminum is separated from lead by liquation and purified by fractional distillation.
In conventional, commercially useful aluminum alloys produced by the Bayer and Hall-Heroult processes, neither the iron nor the silicon content exceeds about 0.1 wt. %. To be commercially competitive, alloys made by the carbothermic direct-reduction processes should have similar iron and silicon levels. In the alloys destined for the aluminum-silicon casting alloy market, however, the Si content can reach 12 wt. % and the iron content may reach 1 wt. %. In alloys contain substantially higher amounts of iron, conventional solidification at cooling rates less than 10.sup.2 K/sec produces severe microsegregation, in which 10-100 micrometer size Al-Fe-Si intermetallic compounds undesirably embrittle the alloy. As a result, when a carbothermically reduced, aluminum alloy contains more than about 0.1 wt. % Fe, the alloy has been further refined employing, for example, dissolution in molten lead to provide a sufficiently ductile alloy that is commercially useful in conventional casting and manufacturing processes. This additional processing increases the costs of the aluminum and the products manufactured therefrom.